Feed URL no longer ends with PULP_MANIFEST for directory synchronization

Change directory synchronization method to no longer expect the feed URL to end with PULP_MANIFEST.  Update the puppet importer to first try to synchronize the repository assuming it is a directory containing a pulp manifest.  When fetching the manifest fails, the importer assumes the feed URL is for puppet forge and retries using the force synchronization method.

Hopefully, Pulp can support multiple importers on a repository in the future so we wont need this sort of thing.